The present invention is related to wireless networks, and in particular to a method and apparatus for transmitting special beacons in a dedicated channel from a wireless station to direct clients to where full beacons are being sent by an access point of a wireless local area network (WLAN). By having a dedicated channel for such beacons, such special beacons may be sent more frequently than ordinary full beacons.
The present invention will be described in terms of the IEEE 802.11 standard, although the invention is not restricted to such wireless networks. In IEEE 802.11 compatible wireless networks, beacon frames are sent out periodically by an access point (AP). Probe responses are sent out in response to probe requests. Beacon frames and probe responses convey information to the associated and associating clients and to other APs as to the properties of the access point transmitting the beacons. They thus provide a mechanism to detect wireless networks.
By passive scanning is meant listening for beacons and probe responses without first transmitting a probe request. Thus, for an AP, passive scanning is the listening for and usually also recording of information from beacons and probe responses from other APs that are transmitting such beacons and probe responses. For a client, passive scanning is the listening for and usually also recording of information from beacons and probe responses from APs other than the client's AP that are transmitting such beacons and probe responses.
By active scanning is meant transmitting a probe request prior to listening for beacons and probe responses. Both active and passive scanning can occur on the same channel used for wireless communication (the “serving” channel) or other channels (“non-serving” channels). For non-serving channels typically an active scan is used. Infull active scanning, the serving channel is vacated to probe all channels. Most wireless network interface devices support a mode usually called monitor mode wherein traffic on all channels is recorded, and this can be used for full active scanning.
Both active and passive scanning, for example, can be used to detect wireless networks. One application, for example, is determining another available network to roam to. Another is detecting potentially rogue access points. See for example, U.S. patent application Ser. No. 10/766,174 file Jan. 28, 2004 to inventors Olson, et al., titled A METHOD, APPARATUS, AND SOFTWARE PRODUCT FOR DETECTING ROGUE ACCESS POINTS IN A WIRELESS NETWORK, assigned to the assignee of the present invention, and incorporated herein by reference.
In the future it is likely that active scanning will not be permitted in certain regulatory domains, hindering the ability of devices to detect wireless networks. More specifically, due to radar detection requirements, devices may not be able to transmit on a given channel until they listen to the medium for a fixed period.
Thus, it may be that only passive scanning will be available to detect other wireless networks. Such passive scanning includes the device listening on each channel, e.g., for beacon frames. Thus, at worst, a device would need to listen for the maximum beacon period. For a battery-operated mobile device seeking to roam, this negatively impacts both roaming speed and battery life.
As an example of an application in which both battery life and roaming speed is important, mobile phone devices are being designed that can operate both on cellular wireless telephony networks, such as GSM, and in a WLAN, e.g., an IEEE 802.11 compatible WLAN. Such devices may be continuously searching for IEEE 802.11 WLANS in order to switch over from the more costly cellular, e.g., GSM network. Battery life, of course, is critical to such devices.
One proposed solution to this is to simply send beacons more frequently, thereby reducing the time a device needs to listen on a given channel. However, beacon frames can be quite large. Furthermore, many new wireless devices are being designed to operate as multiple “virtual” APs, transmitting beacons for multiple infrastructure wireless networks and corresponding network identifiers. The beacons from such devices are even larger than from a single AP device because of the need to send a list of the network identifiers (SSIDs) supported by the AP. Such beacon frames can consume bandwidth that could otherwise productively be used for data. This is exacerbated by the fact that beacon frames are transmitted at the lowest data rate for the basis service set (BSS). Furthermore, each BSSID's beacons need to be sent.
Thus, sending beacons more frequently could have a severe impact on data throughput. For channels being used for voice data, e.g., Voice over IP (VoIP), this will affect both the quality of service (QoS) and, when using call admission control, the number of active mobile devices a channel can support.
Recently, a proposal for “Scheduled Autonomous Probe Response” (SAPR) was made by Motorola, inc. in an IEEE 802.11k Wireless LANs meeting applicable, e.g., for VoIP data. See “IEEE 802.11 04/1010: Proposal and Normative Text for a Scheduled Autonomous Probe Response Generation Function,” Date: Sep. 9, 2004, authors Steve Emeott, et al. The idea is to reduce passive scan latency. The SAPR generation function allows an AP to autonomously transmit Probe Response frames at periodic intervals. The SAPR interval is indicated by a SAPR subfield included in a SAPR information element within Beacon and Probe Response frames. The presence of scheduled, autonomous Probe Response frames with the SAPR information element allows stations to rapidly take measurements and find neighbor APs using passive scanning. Thus, the idea is to reduce the amount of time required for passive scanning clients to “find” an AP. Thus, the purpose of SAPR frames is to increase interactivity.
U.S. patent application Ser. No. 11/156,054 to inventors Kinder et al., filed Jun. 17, 2005, and titled USING MINI-BEACONS IN A WIRELESS NETWORK, describes using minimally configured “mini-beacons” between, and more frequently than full beacons. Such beacons have all but the absolutely essential information elements stripped from it. However, there may still be a need to send full beacon information more frequently, e.g., for roaming operations.
Thus there is still a need in the art for an alternate mechanism for sending beacon frames more frequently in a wireless network.